1. Field of the Invention
The present invention relates to clock transfer devices, which are used in communication and measurement fields in particular and which perform long-distance transfer of reference frequency clock signals generated based on high-precision frequency standards in the form of optical pulse trains toward remote places.
This application claims priority on Japanese Patent Application No. 2006-135156, the content of which is incorporated herein by reference.
2. Description of the Related Art
Present frequency standards are determined based on microwaves (whose frequencies are below 1010 Hz) resonating transitions of cesium atoms. The next-generation high-precision frequency standard is developed based on optical transition of cooling atoms realizing higher frequencies instead of microwave transition. The frequency precision depends upon Δf/f where f (Hz) denotes transition frequency, and Δf (Hz) denotes frequency uncertainty; hence, it is possible to improve the precision of the frequency standard by use of a transition of higher frequencies. By way of the resonation of the optical frequency of a frequency-stabilized laser with the atomic transition, it is possible to use light for the high-precision frequency standard. Since the optical frequency reaches 1015 Hz, it is very difficult for the existing electric circuitry to process the light because of its high-speed performance; hence, it is very difficult for the existing electric circuitry to perform counting.
For the aforementioned reason, mode-locked lasers (or mode synchronized lasers), which output pulse strings at certain time intervals, have been developed and used for performing counting. As shown in FIG. 1, the mode-locked laser forms longitudinal-mode lines aligned with the equal spacing therebetween on the frequency axis; and this is called a frequency comb. The interval of the frequency comb is identical to the repetition frequency of the pulse train (i.e., the reciprocal of time interval) and is represented by an equation (1), where fn (Hz) denotes a single longitudinal-mode frequency of the frequency comb, and fr (Hz) denotes repetition frequency.fn=n fr+fo   (1)
In the above, n is an integer, and fo is an offset frequency of the frequency comb whose frequency is virtually extended toward zero.
First, the frequency comb is stabilized such that the offset frequency becomes zero or becomes identical to a prescribed value; then, the repetition frequency is controlled such that a single longitudinal-mode frequency of the frequency comb becomes identical to the optical frequency of a laser whose frequency is stabilized due to optical transition of cooling atoms. In this state, since the repetition frequency is approximately 109 Hz, it is easy for the electric circuitry to perform counting. According to the equation (1), the repetition frequency precision directly reflects the precision of atomic transition frequency; hence, the pulse train generated by a mode-locked laser at certain time intervals between pulses can be used for a reference clock having the high precision of the atomic transition standard. For example, when the optical transition frequency is set to 500 THz (where n=500000), and the offset frequency is stabilized at zero, it is possible to realize a reference clock whose repetition frequency is 1 GHz. This is taught in the paper entitled “Standards of Time and Frequency at the Outset of the 21st Century” written by S. A. Diddams et al on p.p. 1318-1324 of 19 Nov. 2004 VOL 36 SCIENCE.
The aforementioned frequency comb whose frequency is stabilized due to atomic transition can be used as a high-precision scale in the frequency axis in absolute frequency measurement. Suppose that there occurs a beam having unknown optical frequency. When the beam is overlapped with the frequency comb, it is possible to detect a beat signal whose frequency substantially matches the frequency difference between the unknown optical frequency and the longitudinal-mode frequency of the frequency comb. The electric circuitry is used to measure a beat frequency fb (Hz), thus allowing the unknown optical frequency f (Hz) to be calculated in accordance with an equation (2).f=n fr+fo+fb   (2)
In order to set the offset frequency to zero, or in order to stabilize offset frequency at a certain value, it is necessary for the spectrum band of a mode-locked laser to be broader than one octave (where the double frequency of the low-limit frequency of the spectrum substantially matches the high-limit frequency). Generally speaking, this is a very difficult problem. At the present, only the titanium-sapphire laser is known as a mode-locked laser that can stabilize the offset frequency at a high precision and that can generate reference clock signals or reference frequency combs based on the atomic frequency standard. This is taught in the paper entitled “An Optical Clock Based on a Single Trapped 199H+ Ion” written by S. A. Diddams et al on pp. 825-828 of SCIENCE VOL 293 Aug. 3, 2001.
In order to perform high-precision measurement at a remote place by use of the aforementioned reference clock, it is necessary to perform long-distance transfer of the reference clock. There are provided two methods for the reference clock transfer, i.e., a first method in which a continuous wave optical source is subjected to amplitude modulation based on the reference clock and is then subjected to transfer via an optical fiber network, and a second method in which an optical pulse train generated by a mode-locked laser is directly subjected to transfer via an optical fiber network. Herein, the second method realizes a one-digit higher precision in transfer. This is taught in the paper entitled “Precise frequency transfer through a fiber network by use of 1.5-μm mode-locked sources” written by Kevin W. Holman et al on p.p. 1554-1556 of OPTICS LETTERS, Vol. 29, No. 13, Jul. 1, 2004.
In order to realize transfer of an optical pulse train using quartz fibers, it is necessary to select 1.5 μm wavelength presenting a small transfer loss. At the present, only the titanium-sapphire laser of 800 nm wavelength is known as a mode-locked laser that can supply the frequency standard stabilized as the atomic frequency. A laser of 1.5 μm band may be used for a trial but cannot be used because of technological difficulty. There is reported another technology in which the repetition frequency of a mode-locked laser of 1.5 μm wavelength is actively controlled using an electronic circuit so as to establish synchronization with a reference clock. However, this technology needs a complex electronic control circuit, and it suffers from the occurrence of timing jitters having several tens of femtoseconds due to the operation speed limit of the electronic circuit. This is taught in the paper entitled “Ultralow-jitter, 1550-nm mode-locked semiconductor laser synchronized to a visible optical frequency standard” written by David J. Jones et al on p.p. 813-815 of OPTICS LETTERS, Vol. 28, No. 10, May 15, 2003. This technology realizes only the repetition frequency transfer, whereas the offset frequency is not stabilized; hence, it is very difficult to transfer a frequency comb synchronized with a reference frequency.